1. Field of the Disclosure
The present disclosure relates to a backlight unit for a liquid crystal display device, and more particularly, to a backlight unit for a liquid crystal display device that prevents a light guide plate from moving and blocks light leakage.
2. Discussion of the Related Art
Liquid crystal display (LCD) devices are widely used as monitors of notebook computers, monitors of personal computers and televisions due to excellent reproduction of moving images and a high contrast ratio. LCD devices use the optical anisotropy and polarization properties of liquid crystal molecules of a liquid crystal layer to produce an image.
An LCD device includes two substrates spaced apart from each other and facing each other as well as a liquid crystal layer interposed between the two substrates. The alignment direction of the liquid crystal molecules is controlled by varying the intensity of an electric field applied to the liquid crystal layer, thus changing the transmittance of light through the liquid crystal layer.
The LCD devices require an additional light source because the LCD devices are not self-luminescent. Therefore, a backlight unit is disposed at a rear side of a liquid crystal (LC) panel and emits light into the LC panel to display images.
Backlight units include cold cathode fluorescent lamps (CCFLs), external electrode fluorescent lamps (EEFLs), and light emitting diodes (LEDs) as a light source. Among these, LEDs have been widely used due to their small sizes, low power consumption, and high reliability.
Backlight units are in general classified as edge type or direct type according to the position of the light source with respect to an LC panel. In edge-type backlight units, a light guide plate is disposed under the LC panel, and one or a pair of lamps are disposed at one side or at each of two sides of the light guide plate. In direct-type backlight units, a plurality of lamps is disposed directly under the LC panel, and light from the lamps is directly provided to the LC panel.
The direct-type backlight units are widely used for LCD devices considering brightness as important because of the resulting limitation on the decrease in thickness of the device that is possible, while the edge-type backlight units are widely used for LCD devices considering a thickness as important such as monitors of notebooks or personal computers.
FIG. 1 is a cross-sectional view of illustrating a liquid crystal display (LCD) device including an edge-type backlight unit having LEDs as a light source according to the related art.
In FIG. 1, the related art LCD device includes a liquid crystal panel 10, an edge-type backlight unit 20, a guide panel 30, a top cover 40 and a cover bottom 50.
The liquid crystal panel 10 displays images and includes first and second substrates 12 and 14 facing and attached to each other with a liquid crystal layer (not shown) interposed therebetween. Polarizers 19a and 19b are attached at rear and front surfaces of the liquid crystal panel 10 and control the polarization of light.
The backlight unit 20 is disposed at a rear side of the liquid crystal panel 10. The backlight unit 20 includes an LED assembly 29, a reflection sheet 25, a light guide plate 23 and a plurality of optical sheets 21. The LED assembly 29 is disposed at an edge of at least one side of the guide panel 30 along a length direction. The LED assembly 29 includes a plurality of LEDs 29a emitting light and a printed circuit board (PCB) 29b on which the LEDs 29a are mounted. The reflection sheet 25 is disposed over the cover bottom 50 and is white-colored or silver-colored. The light guide plate 23 is disposed over the reflection sheet 25. The plurality of optical sheets 21 are disposed over the light guide plate 23.
Edges of the liquid crystal panel 10 and the backlight unit 20 are surrounded by the guide panel 30 having a rectangular frame shape. The top cover 40 covers edges of the front surface of the liquid crystal panel 10, and the cover bottom 50 covers a rear surface of the backlight unit 20. The top cover 40 and the cover bottom 50 are combined with the guide panel 30 to form one united body.
Various optical designs are considered to provide a surface light source from the backlight unit 20 of the LCD device to the liquid crystal panel 10. Among these, maintaining an optical gap A between the light guide plate 23 and the LED assembly 29 is a significant factor.
To do this, a guide groove (not shown) is formed at an edge of the light guide plate 23, and a stopper (not shown) corresponding to the guide groove is formed at the guide panel 30. The optical gap A is maintained by inserting the stopper into the guide groove and preventing the light guide plate 23 from being moved.
However, it is not easy to form the stopper because the guide panel 30 needs to have a thin thickness in realizing an LCD device having a thin profile, light weight and narrow bezel, which have been recently required. In addition, since the guide groove (not shown) at the edge of the light guide plate 23 is close to the active area for displaying an image, light is diffused or scattered by the guide groove (not shown) of the light guide plate 23 when the light emitted from the LEDs 29a progresses in the light guide plate 23 and is refracted and reflected toward the liquid crystal panel 10 to provide a surface light source.
That is, light leakage occurs due to the guide groove of the light guide plate 23, and an area where the guide groove is formed is shown relatively bright as compared with other areas. Thus, problems of lowering qualities of the LCD device such as decreases in brightness and image qualities are caused.